That strange fuzzy feeling you get on your teeth when eating certain foods and drinks come from acids, and all acids can create a weird feeling in your mouth to a certain degree. Wines and teas are famous for having tannic acids, strawberries have malic acid, and spinach and bananas both have oxalic acids. There are many other examples of this phenomena, and some of these dietary acids can be bad for your tooth enamel.
Lately, I have been fielding a lot of questions on Quora from people concerned about how the oxalic acids in spinach might be affecting their teeth. While it is true that oxalic acid has the potential to negatively effect your teeth because of its acidic properties and ability to chelate calcium, it is highly unlikely that oxalic acid is detrimental to your teeth in the form that it is present in most foods. Let's talk about why.....
Oxalic Acids in Spinach
When thinking about oxalic acids the food that most often comes to mind is spinach. The oxalic acid content in spinach is relatively high (~6,580 ppm), which causes many people to be concerned. However, the overall pH of Spinach is not very acidic (cooked = 6.6) and spinach also has high calcium and iron content. It is also important to keep in mind that the oxalic acid content also depends on how the spinach is prepared; cooking/boiling spinach will reduce the oxalic acid content.
Relative Acidity of Spinach
Even though the oxalic acid in spinach behaves as a strong acid, since the pka(s) of oxalic acid are 1.38 and 4.28, the overall pH of (cooked) spinach is fairly close to neutral meaning that it is highly unlikely to erode your enamel. Additionally, the high calcium concentration in spinach (though most of it may be highly bound to oxalate and not available in solution) would make your enamel even less likely to dissolve.
The calcium chelation properties of oxalic acid, which would cause some demineralization of your enamel, are inhibited by calcium and iron concentrations in the spinach. The oxalic acid, in the form of oxalate, has already tightly bound to the iron and calcium in spinach (which is probably why it has high concentrations of these minerals to begin with), and as a result is not free to remove calcium from your enamel. Chelation is also a pH dependent process, and since your oral pH is similar to the pH of (cooked) spinach, the oxalic acid is likely already in equilibrium. As a side note, spinach isn't a very good source of iron or calcium because the minerals are so tightly bound to oxalate that they are unavailable to be absorbed in the gut.
Oxalic acid/oxalate is also known to strongly bind magnesium and potassium among other things, which means that chelation of calcium is likely not a danger in bananas either (though the sugars can still pose a problem).
The calcium binding properties of the oxalates found in spinach actually cause them to precipitate as crystals on the surfaces of your teeth, which causes the fuzzy feeling you get when eating them. Interestingly, this is the same principle behind many topical treatments for tooth sensitivity you receive from your dentist and these types of foods can help plug up the dentinal tubules where the pain starts. Eating oxalate rich foods is just one of many changes you can make in your diet to reduce tooth sensitivity.